Vapor lock or fuel pump problem?

Hi everyone!

I just did a 10-minute research thru the forums and didn't see anyone post this particular problem I'm having.

So, for the past year I've been experiencing a peculiar problem where on the hwy I'll speed up to pass a line of traffic and I'll hit maybe 80 mph or so then the engine starts acting like it's running out of gas. For the next 15-20 seconds I'll be losing speed and pull over to the shoulder. The more gas I try to give it, the worse it gets but after I release the throttle for about 5 seconds while coasting in gear, it'll have power when I do get back on the throttle. So, it seems as though I've got my own governor: if I hold the throttle at 90% or more for more than 10 seconds, I lose power for some time. The bike will maintain 70 mph no problem on a flat surface. However if I'm going up a hill and I need to give more gas to maintain that speed, it's starts bogging down again.

My thought is to disconnect the hose from the gas cap and blow it out with an air compressor. After that, I have no idea what to do. Are there problems with the '99 model's fuel pump? Any ideas or has anyone else experienced this?

Thanks!

I would first suspect a restricted fuel filter if you haven't changed it. Secondly is the vacuum auto fuel shut-off is gradually failing. That device is just ahead of the filter near back right side of motor....can't miss it.

Replace the fuel filter first and go ride to test.
If it fails again then route around the fuel cutoff using the existing tubing (no cost), be sure to block the vacuum line as well. Then go test again.
I'm putting my money on the fuel cutoff...

Mark said:

Replace the fuel filter first and go ride to test.
If it fails again then route around the fuel cutoff using the existing tubing (no cost), be sure to block the vacuum line as well. Then go test again.
I'm putting my money on the fuel cutoff...

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+1!......my filter suggestion first.....as I'm sure it needed done anyways.....and FWIW, use an OEM one, not aftermarket......later, down the rode, your fuel pump WILL thank you, as will your wallet.

Since the fuel filter is downstream from the pump output on a ST1100, I don't understand what effect the fuel filter will have on the pump.

I would change the filter first, also. And while you have the "shelter" off , it's easy to check the fuel pump output.

If you have a flakey fuel pump, you can install a LOW PRESSURE automotive pump.

X2 on filter and vacuum fuel gauge. Bypass the gauge to diagnose, then rebuild the gauge or just leave it off like many others do. OE filters are nice, but there are many that will work just as well. In my oppinion of course.......Rod

I would say it is the vac. fuel valve. About a 30 minute job, start to finish.

Jim Van said:

Since the fuel filter is downstream from the pump output on a ST1100, I don't understand what effect the fuel filter will have on the pump.

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It's not that the filter will effect the pump... but, it can (and has) restricted the flow of fuel, starving the bike...

Mark said:

It's not that the filter will effect the pump... but, it can (and has) restricted the flow of fuel, starving the bike...

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......and can increase electrical resistance load on the pump, decreasing its life.

^^^^ Mark and Jim are correct. The pump is a pressure relief / bypass style. So, NO.....

Mark said:

It's not that the filter will effect the pump... but, it can (and has) restricted the flow of fuel, starving the bike...

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I don't want to start an argument, but I seriously doubt that. I have been using auto fuel filters for years without a problem. There is nothing magic about the filter element inside a Honda fuel filter, they just cost about 3 times more than they should.

No argument Jim.
I was commenting on the fuel being restricted, not on the pump being damaged.

I don't have an opinion on that but Adam and Brant do!

afrymoyer said:

^^^^ Mark and Jim are correct. The pump is a pressure relief / bypass style. So, NO.....

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Thanks!.......good to know.........and elimiates my previous thought of that being a possible cause of a premature pump failure.

Do the petcock removal and might as well change fuel filter. You bike has the exact same symptoms my bike had. While I changed both at once I'll never really know if it was the fuel filter or petcock, but I believe (I want to believe) it was the petcock.

The fuel valve isn't functioning correctly. It is not allowing a full flow of gas. Under heavy acceleration or extended high RPMs, the carb bowls run low on gas. Starts cutting out, running rough, or what ever you want to call it. By pass is relatively easy and quick. Reversing back is easy. Don't have to put all the plastic back on to test ride it.

Maybe the filter, but I doubt it.

Thanks, everyone! I'll give these suggestions a shot...starting with the easiest one!

Carb bowls have fuel, enough to handle a hard acceleration/prolonged passing. It's getting warmer and more humid on the east coast- I would sample the fuel system for water.

And try some 90% rubbing alcohol. About 10 oz. in a full tank. That will take care of any water that may be present.

Jim Van said:

And try some 90% rubbing alcohol. About 10 oz. in a full tank. That will take care of any water that may be present.

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Not........the fuel is already 10%+ alcohol....adding more just draws more miosture in tank......that's 'old school' and doesn't apply with fuel of today. If you want to add anything, use Star-Tron enzyme fuel treatment for a stabilizer, eliminating water/fuel molecule seperation.

The fuel filter is fairly new so I don't think it is that and I always use Star-Tron fuel treatment at every fill-up. So, I just took a long ride after bypassing the fuel valve. There was no evidence of the problem I mentioned earlier. No bogging, no lack of gas and the bike "carbureted" as it was if it was new. Therefore, the fuel valve is the culprit. Which leads me to my next bunch of questions:

What does this fuel valve do? How important is it? Is it necessary to replace it? I usually side with the thought "the Honda engineers put it there for a reason", but I have been convinced before that sometimes little components can be eliminated.

Thanks!